In order to tackle stubborn dirt, we all too readily turn to the heavy-duty means: sandblasting, corrosive products and solvents, high-pressure cleaning, sanding, brushes etc.
These solutions are far from ideal, as they are often aggressive, harmful to the substrate, downright dangerous, or even environmentally polluting. Yet there is a solution that is effective, efficient and virtually universally usable: dry ice blasting.
Dry ice blasting uses solid CO2 pellets as the particles and a carrier gas, preferably compressed air, to accelerate the pellets in the direction of the surface to be treated.
A dry ice device is generally composed of a feed hopper filled with particles, a mixing device for mixing the particles with the carrier gas, and one or more discharge means to which a gun can be connected in order to project the particles towards the surface to be treated. In most cases the mixing device will regulate the particle consumption.
There are various types of mixing devices for such dry ice devices. A commonly used type is the driven rotor with holes. Such a type is disclosed in European patent application EP 1 340 592. According to this patent application, the mixing device comprises a disc rotatably disposed between two non-rotatable plates and rotatable about a vertical axis. The disc has two functions:    1) to accommodate the particles present in the feed hopper in the holes of the disc rotating below the top plates;    2) to bring the filled holes into contact with a carrier gas.
Varying the speed of rotation of the disc can regulate the particle consumption.
Another type of mixing device used is disclosed in the international patent publication WO 03089193. According to this patent publication, the mixing device comprises a cylinder, which on its outer circumference is provided with cavities or recesses. The cylinder is rotatable about a horizontal axis and is disposed between a top chamber where the openings are filled with particles from the feed hopper and a bottom chamber where the particles are mixed into the carrier gas flow. The particle consumption can likewise be regulated by varying the speed of rotation of the disc.
Since the design of the known devices is fairly complex, one of the great challenges of the known systems is to achieve a good seal between the rotating rotor and the non-rotatable discs or chambers. The device wherein the rotor is rotatable about a vertical axis makes use of sealing discs disposed in a stationary position above and below the rotor, and partially or fully covering the rotor. The pressure at which the seal is produced can be regulated by means of springs or sealing elements, which make the construction of the whole unit even more complex.
The seal of the mixing devices with a rotor that is rotatable about a horizontal axis has to be provided on the outer circumference, which results in a very complex construction of the top and bottom chamber, and in high motor torque. In order to maintain the seal at raised pressure and without a high motor torque, complex bottom chambers with flexible inside walls and various seals have already been developed. SUMMARY OF THE INVENTION
The object of the invention is to provide a device for particle blasting wherein the seal of the mixing device can be achieved in a simple manner.
The object of the invention is achieved by providing a device for particle blasting comprising:                first supply means for a carrier gas;        second supply means for particles;        a mixing device provided for mixing the particles and the carrier gas, comprising a rotatable distribution disc with one or more cavities, which is provided in order to bring the particles into contact with the carrier gas;        discharge means provided for discharging the mixture formed;wherein the mixing device comprises a mixing plate provided with at least one supply channel for the carrier gas and at least one discharge channel for the mixture, and wherein the rotatable distribution disc is provided in order to position the cavities in such a way during rotation that they form a temporary connection between the at least one supply channel and the at least one discharge channel. The great advantage of the device according to the invention is that the device need only be sealed in one plane. The seal of the rotatable distribution disc and mixing plate is much simpler, since the rotatable distribution disc is exposed on only one side to the carrier gas under pressure, and therefore has to seal only on one fixed element. Where the known devices use two non-rotatable elements that are in contact with the particles and the rotatable distribution disc, the device according to the invention has only one non-rotatable element, namely the mixing plate.        
Since both the supply channel and the discharge channel provided are situated in one and the same element, a very simple arrangement is obtained, and said arrangement has yet another advantage. It is namely that, if the pressure of the carrier gas gets lost during rotation of the distribution disc, in the known devices, as a result of the force of gravity, the inlet and/or outlet of the carrier gas channel will be filled with particles. If the particles are solid CO2 pellets, it can happen that the inlet and/or outlet of the channel is/are blocked as a result of the frozen pellets. In the device according to this invention there is no risk of the inlet or outlet becoming blocked with pellets if the pressure of the carrier gas gets lost and the distribution disc continues to rotate, since the pellets will not leave the cavities in the distribution disc as a result of the force of gravity.
The carrier gas used in the device according to the invention is in particular compressed air, but it can also be another known carrier gas. The particles are preferably CO2 pellets or a mixture of CO2 pellets and another material such as, e.g., silicates, salt crystals and the like. The particles can be supplied to the device in the form of scrapings (e.g. from blocks), flakes or powder.
In a preferred embodiment of the device according to the invention a dividing wall is provided between the supply channel and the discharge channel, and the channels are provided in order to achieve the said connection at the level of the dividing wall.
According to a more preferred embodiment of the device according to the invention, the mixing device comprises a non-rotatable sealing plate, which is disposed between the mixing plate and the rotatable distribution disc. In a preferred embodiment the said non-rotatable sealing plate is composed of a plurality of layers, so that, inter alia, the layer that could possibly be subject to wear is easy to replace.
According to a special embodiment of the device according to the invention, the said sealing plate comprises at least one first aperture, which is in communication with the second supply means, and the said sealing plate comprises at least one second aperture at the level of the dividing wall, the second aperture connecting to both the supply channel and the discharge channel.
According to a preferred embodiment of the device according to the invention, the mixing device comprises cutting means for reducing the size of the particles fed in. The cutting means are preferably disposed between the mixing plate and the rotatable distribution disc.
In a particularly advantageous embodiment of the device according to the invention the rotatable distribution disc comprises at least one series of cavities placed at regular intervals and at equal distances relative to the centre point of the disc, which cavities during rotation of the distribution disc form a temporary connection between the at least one supply channel and the at least one discharge channel. During rotation of the distribution disc the cavities placed at regular intervals and at equal distances relative to the centre point of the disc will preferably connect alternately to the first and second apertures. In a special embodiment of the device according to the invention the number of cavities per series is an odd number. If this is combined with two filling apertures, two first apertures and two second apertures which give access to the discharge channel, it results in uniform mixing of the particles with the carrier gas and, furthermore, the undesirable pulsing (jolting) effect is reduced.
Even better mixing of the particles with the carrier gas is obtained in a special embodiment of the device according to the invention, wherein the rotatable distribution disc comprises a first series and a second series of cavities, the said series of cavities being placed at a first and a second distance respectively from the centre point of the disc. In particular, the first and second series of cavities are in an offset position relative to each other.
According to a preferred embodiment of the device, the device comprises pressure means for pressing the rotatable distribution disc against the mixing plate. The said pressure means preferably press the rotatable distribution disc against the mixing plate along one side.
In particular, the said pressure means comprise a pressure chamber, and the pressure exerted by the pressure chamber is proportional to the pressure of the carrier gas. This has the advantage that a pressure-dependent seal is achieved and unnecessary friction and subsequent wear at lower pressures is avoided. In a preferred embodiment the said pressure chamber is situated outside the mixing device.
In a most special embodiment of the device according to the invention the said device is a dry ice blasting device.
In order to explain the features of this invention further and to indicate additional advantages and details of it, there now follows a more detailed description of a device for pellet blasting according to the invention. It should be clear that nothing in the description that follows can be interpreted as a limitation of the protection for the device according to the invention demanded in the claims.
In this description reference is made by means of reference numerals to the appended drawings, wherein: